Add methods "Pi.read" and "_callback_handler.remove"

[Wesitos]

Before this changes, trying to cancel a callback would raise an exception (because the "remove" method of the "_callback_handler" class wasn't implemented. This also makes "Callback.cancel" a coroutine.

Also, I implemented the "Pi.read" method. Was there any reason to not implement it?

[PierreRust]

Sorry for the delay, I somehow managed to miss your PR. No particular reason for omitting the read method, apigpio is still very incomplete and I tend to implement methods from the original pigpio only if and when I need them.